This invention relates to a semiconductor infrared laser, and more particularly to a higher power Pb.sub.1-x Sn.sub.x Te diode infrared laser with substantially complete infrared frequency coverage. It also involves an improved method of making such a laser.
The band gap energy of a semiconductor material determines the photon energy of semiconductor diode radiation. Accordingly, the wavelength of the semiconductor diode radiation is a function of the semiconductor material band gap. A laser internally collects such radiation and amplifies it by using the collected radiation to stimulate additional radiation. Since the additional stimulated radiation is from the same semiconductor diode, it is of the same wavelength. Hence, the wavelength of radiation emitted from a semiconductor laser is essentially a function of the semiconductor material band gap.
A semiconductor laser is usually formed in a monocrystalline body having a PN junction and two mutually parallel reflective faces perpendicular to the PN junction. The body is usually a parallelepiped, and the reflective surfaces form a laser cavity adjacent one side of the PN junction. However, the cavity does not have to be formed in a parallelepiped body, or even in a body with flat parallel faces. Ring-type lasers, cylindrical lasers, and others are known.
The lasing action is produced by applying a voltage across the PN junction. Radiation emitted from the PN junction due to the voltage applied is collected and amplified in the laser cavity. The amplified radiation exits the laser cavity parallel to the PN junction as a monochromatic and coherent beam. The radiation wavelength emitted by a laser, as previously mentioned, is essentially a function of the semiconductor material band gap. Composition of the semiconductor material is the most important factor that determines the band gap. However, it is not the only factor. Laser body operating temperature, magnetic fields and pressure also affect the band gap. They can be used to precisely adjust the principal active radiation mode of a laser to a preselected wavelength. Such adjustment is referred to herein as tuning.
Pb.sub.1-x Sn.sub.x Te semiconductors have band gap energies which make them ideally suited for lasers that emit infrared radiation of a wavelength from about 6.5 microns to 32 microns. In the past, Pb.sub.1-x Sn.sub.x Te lasers could not be made with high output power for all infrared frequencies, as for example wavelengths of 6.5 - 9 microns. This particular range is of interest for spectroscopy applications.
Imperfections in the PN junction and contiguous regions of the semiconductor diode laser are highly undesirable. They produce both electrical and optical losses that reduce output powder of the preselected principal active radiation mode. In lead-tin telluride bodies, it is difficult to obtain a satisfactory PN junction that has contiguous regions with satisfactory carrier concentrations, mobilities and dislocation densities.
Various techniques have been used in the past to produce a PN junction in a lead-tin telluride body, including impurity diffusion from an external source. The impurity diffused PN junction was formed at a relatively high temperature; e.g., above 700.degree. C. Cooling the monocrystalline body from such a high temperature treatment such as this can generate crystal dislocations. Crystal dislocations can cause inferior PN junctions and reduce current carrier mobility, resulting in lower output power in the laser. Self-diffusion procedures have also been used to form the PN junction. They may have involved somewhat lower treatment temperatures. However, the lasers thus formed were limited in output power for some radiation wavelengths, as for example 0.1 milliwatt at 6.5 - 9 micron wavelengths.
I have now found that cadmium rapidly diffuses into Pb.sub.1-x Sn.sub.x Te at a relatively low temperature. It produces low carrier concentration, high mobility N-type regions that, if thin, form excellent PN junctions for a semiconductor laser. The cadmium is diffused into the crystal from an external source. Hence, full variation in semiconductor composition is available, which permits one to tune the laser to the full spectrum of infrared wavelengths from 6.5 microns to 32 microns while maintaining higher output power. In addition, the low temperature cadmium diffusion minimizes any increase in crystal dislocations in cooling after diffusion, and any power losses directly or indirectly attributable to them. Accordingly, a more tunable, higher output power infrared laser can be obtained. By higher output, I mean output of the order of 1 milliwatt. Other advantages are also obtained, since only a short duration heat treatment is needed to obtain sufficient cadmium diffusion.